This invention relates to DC link, variable speed constant frequency (VSCF) power systems having at least two parallel connected channels, and more particularly, to a method and circuit for controlling the DC content in the output of such systems and for selectively isolating channels which contribute excessive DC content to the parallel system.
DC link VSCF electrical systems include a generator which is driven at a variable speed, and produces a variable frequency AC output voltage. This AC output voltage is rectified and fed to a pair of DC link conductors. An inverter receives voltage from the DC link conductors and converts it to a constant frequency DC output. Electronic switches in the inverter are paired so that conduction by one switch in a pair generates the positive half cycle of the output current waveform and conduction by the other switch in the pair generates the negative half cycle. In order to generate an ideal waveform, each switch pair must be on for one-half of the total duration of each cycle of the output waveform and off for one-half of the time. However, due to variations in the characteristics of the switches, such as switching time and saturation voltage, it is inevitable that one switch in a pair will generate more volt-seconds per cycle than the other. Although this difference in volt-seconds generated is usually very small, over a period of time, it results in an introduction of a DC component to the output waveform Techniques and circuits for sensing this DC component and modifying the inverter switching to minimize DC content are illustrated in U.S. Pat. Nos. 4,500,837 and 4,370,702. Although those circuits provide suitable DC content correction and control for single channel systems, parallel connected VSCF systems present special situations which are not adequately handled by existing DC content control circuits.
In particular, it has been found that splitting the channels of a parallel VSCF system as a result of excessive DC content in the output, did not guarantee that only the culprit channel would be tripped. To understand this problem, it is instructive to consider a parallel VSCF system consisting of many VSCF channels. When the system is running in a parallel configuration, all of the channels connected to a parallel bus will try to bring the DC content at their sensing points (normally the point of regulation) back to their pre-paralleled DC content levels. The actual DC contents sensed by individual paralleled VSCF channels will be different if there are interconnecting feeders between the various points of regulation. Therefore, each parallel channel will try to regulate its post paralleled DC content at its sensed point of regulation to the pre-paralleled value.
If one of the VSCF channels has a high, but below trip threshold, DC content before it is connected to the paralleled bus, that channel would attempt to introduce a higher DC content on all of the good channels because of the low DC resistance of connecting feeder cables. The good channels would then try to reduce their respective DC content back to their pre-paralleled values. Since existing DC content correction loops have infinite gain, when attempting to regulate the DC content, some of the DC content correction circuits in the good channels may saturate. Eventually, an equilibrium would be reached and none of the paralleled channels would have a high enough DC content to warrant tripping. Therefore, the system would continue to run in the paralleled mode.
Now, if the channels of the system are to be split or a good channel is to be taken off the paralleled bus, the following scenario may occur. The good channels with the post-parallel saturated DC content controls would take some time to recover from saturation. During that time, those channels would cause maximum DC content at their sensing points. During recovery of the DC content correction loops, the DC content at the sensing points will be driven to the dynamic limit of the DC content correcting loop. Depending upon the degree of saturation, the time taken by the saturation channels to recover may exceed the trip threshold resulting in tripping of the otherwise good channels.
Another fact situation can also result in the tripping of good channels. For example, suppose that one channel on the paralleled bus has developed a problem in its DC content control. This problem, had it occurred in the isolated split channel mode, would have caused a DC content beyond the trip threshold and subsequent tripping of that channel. In the parallel mode, this bad channel would cause a higher DC content at the sensing points of all good channels because of the low DC resistance of the interconnecting feeder cables. The good channels would then try to regulate their higher post-parallel DC content to their pre-parallel values. When correcting the DC contents of their channels, the good channels may reduce the DC content on the culprit channel sensing point to below the trip threshold, and in the process some of the good channel DC content correction circuits may be saturated. This scenario is the same as discussed above, except that when the channels are split, the culprit channel would definitely trip and some of the good channels may also trip. Alternatively, the good channels may reduce the DC content on the culprit channel sensing point, but not to a level below the trip threshold, and in the process most of the good channels will have their DC content correction control saturated, with the DC content at their sensing points possibly beyond the trip thresholds. In this instance, an out-of-limit DC content will exist on at least one channel sensing point. If the selective isolation of the culprit channel cannot be implemented, the system will be forced to split and all channels will operate in the isolated mode. Here again, the culprit channel will definitely trip and some of the good channels may also trip.
It can therefore be seen that upon taking a good channel off of a paralleled bus, that channel could falsely trip because of a saturated DC content control. Furthermore, when splitting a paralleled system because of excessive DC content, not only the culprit channel, but also some of the good channels may be tripped. It is therefore desirable to provide a DC content control circuit which permits selective tripping of defective channels in a parallel connected VSCF power system.